In an exhaust purification device of an internal combustion engine arranging an NOX selective reducing catalyst in an engine exhaust passage and feeding aqueous urea stored in an aqueous urea tank into the NOX selective reducing catalyst so as to selectively reduce the NOX contained in the exhaust gas with the ammonia generated from the aqueous urea, known in the art is internal combustion engine arranging an aqueous urea concentration sensor in the aqueous urea tank for detecting abnormalities in the aqueous urea (for example, see Japanese Patent Publication (A) No. 2005-83223).
However, this aqueous urea concentration sensor is expensive. Currently, it is desired to use a more inexpensive alternative method.